Many military and commercial mobile platforms have numerous wireless systems for transferring information from one platform to another and with ground-based systems. These mobile platforms include aircraft, spacecraft, satellites, and many other devices. When one of the onboard wireless systems transmits a signal, part of the energy radiated from the antenna may couple to the antenna terminals of a nearby receiver, particularly a receiver operating at a frequency near the frequency of the transmitted signal. The resulting induced energy (undesired signal) in the receiving circuit will cause spurious responses and thereby prevent the receiver from detecting the signal which it is intended to receive.
As demand for connectivity increases these problems will become more pronounced and troublesome. For instance, as the traveling public takes increasing advantage of mobile Internet services (such as ConnexionSM offered by the Boeing Company of Chicago, Ill.), it is envisioned that additional transceivers may be placed on many mobile platforms. Such systems are also needed on many types of military and commercial platforms, which include helicopters, fixed-wing aircraft, ships, and ground vehicles. Thus, an increasing number of systems are likely to become susceptible to interference from nearby transmitters. In particular, the current emphasis on small-unmanned aerial vehicles (UAVs) creates a need for filtering systems that can be integrated with these small airframes. Unfortunately, the size and weight of the previous filtering systems makes it impossible to integrate these systems onboard a UAV. Thus, previous attempts to solve the interference situation have, heretofore, proven unsatisfactory.
The previous techniques used for achieving compatibility between competing signals include: frequency management, analog filters, cryogenic filters, and even active cancellation systems. Frequency management is an administrated process requiring the operator to avoid simultaneously operating systems at the competing signals. While frequency management avoids degrading system performance, and avoids the addition of filters with their attendant weight penalties, frequency management also includes disadvantages. One disadvantage is that limiting the spectrum of frequencies available to the transceiver is limited. In the alternative, transceivers that share a frequency may only be operated sequentially therefore causing a loss of critical information at times. Additionally, the radio operator suffers an increased workload due to having to actively managing the system.
To avoid the complications required by frequency management, other attempts have used combinations of filters and amplifiers to suppress the undesired signals. The amplifiers are inserted between filter stages to compensate for the degradation of the desired signal caused by its pass through the filters. While some suppression of undesired signals may be realized by this technique, these systems are limited to applications in which the competing frequencies are not close together and in which the weight of the filters can be tolerated (i.e. not on mobile platforms). Also, as the inline insertion loss associated with the filters increases, the frequency separation between the desired and undesired signal increases.
The use of cryogenic liquids to cool the filters increases the filter efficiency to the point where the need for inter-stage amplifiers is eliminated. The elimination of the amplifiers is possible because the insertion in the pass band is minimal and the frequency separation between the desired and undesired signal is smaller when compared to the more non-cryogenic filters previously discussed. Thus, system performance is improved somewhat as compared to systems using non-cryogenic filters. However, these systems require heavy cryogenic components along with operational complications caused by the need to maintain cryogenic temperatures during operation (e.g. the filters take some time to cool down and the generation of even small amounts of heat in the filters can vaporize the cryogenic liquid).
Active cancellation systems have also been employed to alleviate the presence of the undesired signals. Active cancellation systems use amplifiers and phase shifters to cancel the undesired signals. Unfortunately, though, each receiver and the interfering transmitter(s) must be interconnected to the cancellation system. Thus, the interconnection creates a single point failure that can deny the use of all receivers connected to the cancellation system. Worse still, if a new transceiver system is added to the vehicle, the interconnection system must be redesigned and grows factorially in weight, size, and unreliability.
Thus a need exists to eliminate the interference caused by closely spaced antennas in a cost, power, and weight efficient manner without occupying undue space onboard mobile platforms.